The present invention relates to a combustor for burning fuel in compressed air. More specifically, the present invention relates to a combustor in which fuel is introduced into two pre-mixing passages by a single fuel distribution ring.
In a gas turbine, fuel is burned in compressed air, produced by a compressor, in one or more combustors. Traditionally, such combustors had a primary combustion zone in which an approximately stoichiometric mixture of fuel and air was formed and burned in a diffusion type combustion process. Additional air was introduced into the combustor downstream of the primary combustion zone. Although the overall fuel/air ratio was considerably less than stoichiometric, the fuel/air mixture was readily ignited at start-up and good flame stability was achieved over a wide range of firing temperatures due to the locally richer nature of the fuel/air mixture in the primary combustion zone.
However, use of such approximately stoichiometric fuel/air mixtures resulted in very high temperatures in the primary combustion zone. Such high temperatures promoted the formation of oxides of nitrogen ("NOx"), considered an atmospheric pollutant. It is known that combustion at lean fuel/air ratios reduces NOx formation. However, achieving such lean mixtures requires that the fuel be widely distributed and very well mixed into the combustion air. This can be accomplished by introducing the fuel into the combustion air in a number of annular pre-mixing passages so that the fuel and air are pre-mixed prior to their introduction into the combustion zones. According to one approach, the fuel is introduced into each pre-mixing passage by separate fuel manifold rings around which fuel discharge ports are distributed and that are disposed adjacent the passage inlets, as shown in U.S. Pat. No. 5,361,586 (McWhirter et al.). Although this approach allows the flow of fuel to each passage to be individually controlled, unfortunately, it results in considerable blockage of the flow area for the combustion air.
Alternatively, fuel can be introduced into two pre-mixing passages by elongate fuel spray tubes, or "pegs," each of which discharges fuel into both passages. A combustor of this type is shown in U.S. Pat. No. 5,479,782 (Parker et al.), hereby incorporated by reference in its entirety. Although such an approach can reduce the blockage of the combustion air flow area, unfortunately, it does not allow the amount of fuel introduced into each of the pre-mixing passages to be individually regulated. This lack of control prevents optimization of the combustion dynamics and can result in excessive combustion instability and noise.
It is therefore desirable to provide a combustor, such as that suitable for use in a gas turbine, in which the flow of fuel to multiple pre-mixing passages can be individually controlled without excessively blocking the combustion air flow area.